Change hash_64() return value to 32 bits

That's all that's ever asked for, and it makes the return
type of hash_long() consistent.

It also allows (upcoming patch) an optimized implementation
of hash_64 on 32-bit machines.

I tried adding a BUILD_BUG_ON to ensure the number of bits requested
was never more than 32 (most callers use a compile-time constant), but
adding <linux/bug.h> to <linux/hash.h> breaks the tools/perf compiler
unless tools/perf/MANIFEST is updated, and understanding that code base
well enough to update it is too much trouble.  I did the rest of an
allyesconfig build with such a check, and nothing tripped.

Signed-off-by: George Spelvin <linux@sciencehorizons.net>

include/linux/hash.h

@@ -48,7 +48,7 @@
 #define GOLDEN_RATIO_32 0x61C88647
 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
 
-static __always_inline u64 hash_64(u64 val, unsigned int bits)
+static __always_inline u32 hash_64(u64 val, unsigned int bits)
 {
 	u64 hash = val;
 
@@ -72,7 +72,7 @@ static __always_inline u64 hash_64(u64 val, unsigned int bits)
 #endif
 
 	/* High bits are more random, so use them. */
-	return hash >> (64 - bits);
+	return (u32)(hash >> (64 - bits));
 }
 
 static inline u32 hash_32(u32 val, unsigned int bits)
@@ -84,7 +84,7 @@ static inline u32 hash_32(u32 val, unsigned int bits)
 	return hash >> (32 - bits);
 }
 
-static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
+static inline u32 hash_ptr(const void *ptr, unsigned int bits)
 {
 	return hash_long((unsigned long)ptr, bits);
 }